FF isn't a replacement for any of them it's another way of doing the same or similar thing.
The one thing that you have to get to grasp with is the way of thinking for responsive designs, you have to take into consideration the content of your pages not only at larger viewports but also the smaller ones (which becomes restrictive because of the size obviously), decide what's important for your visitors, what's the priority for their viewing and for your rerason for a web-site (info/products etc). Once you have got that thought process working, draw out a design on a piece of paper (either with the design at what represents full size or small size. Then lay over the top of that a piece of tracing paper with rows & columns (the columns divided into 12 equal dimensions (thats a start for you to visualise the Grid system and work out how many spans each part of your design might require to follow the layout.
(thats a very simplistic approach but I found one that helped me right in the beginning.
Look at some of the ready made templates in RSD mess about with them,,, change them play and look at the desgn and the way they were constructed,,, take a couple of weeks mimnimum getting to know your way around instead of jumping straight in & trying to design a site that's real.
I don't have the latest RSD V2 but I do have a couple of video's from the earlier versions which might help you around the bits & pieces what goes where and what stuff is used for. I also know there are some video's on Youtube which others have made which you might find useful (google "coffee cup responsive design" (video) you will get quite a lot of results which might prove beneficial also.
Good luck & happy designing for the future
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